Wireless communication systems have become increasingly popular. These applications, however, pose two special problems. First, the Radio Frequency (RF) carrier modulation by which information is transmitted must demand the smallest bandwidth possible due to the general shortage of available spectrum. As a result, both the amplitude and the phase (i.e., frequency) of the carrier must be precisely controlled during modulation. Amplifying the modulated carrier without excessive distortion in the transmitter output stage imposes significant linearity constraints on the output stage amplifier.
Second, the power efficiency of the transmitter is very important where the transmitting end of the wireless link is battery powered. Typically, the transmitter output stage is the largest power consumer; hence, improvements in this stage are the most important. Class A RF amplifiers provide a distortion-free output, but require large amounts of power. Non-linear amplifiers, including class B and class C amplifiers, provide higher RF output power and better efficiency. One of the most efficient RF power amplifiers is the class C RF amplifier in which the output transistor conducts current only at the time when the collector-emitter voltage is at its lowest value. Unfortunately, these amplifiers are very nonlinear and introduce substantial amplitude distortion. Because of this distortion, class C amplifiers are used mainly in FM transmitters in which the amplitude or “envelope” of the RF carrier is constant, and hence, such distortion has no effect on the output spectrum occupancy.
Technologies such as Radio Frequency Identification (RFID) use amplitude modulated signals.
As shown in FIG. 1, an RFID system 100 includes RFID tags 102, a reader 104, and an optional backend system, e.g., server 106. Each tag 102 includes an IC chip and an antenna. The IC chip includes a digital decoder needed to execute the computer commands that the tag 102 receives from the tag reader 104. In some tags 102, the IC chip also includes a power supply circuit to extract and regulate power from the RF reader; a detector to decode signals from the reader; a backscatter modulator, a transmitter to send data back to the reader; anti-collision protocol circuits; and at least enough memory to store its EPC code.
Communication begins with a reader 104 sending out signals to find the tag 102. When the radio wave hits the tag 102 and the tag 102 recognizes and responds to the reader's signal, the reader 104 decodes the data programmed into the tag 102 and sent back in the tag reply. The information can then be passed to the optional server 106 for processing, storage, and/or propagation to another computing device. By tagging a variety of items, information about the nature and location of goods can be known instantly and automatically.
Many RFID systems use reflected or “backscattered” radio frequency (RF) waves to transmit information from the tag 102 to the reader 104. Since passive (Class-1 and Class-2) tags get all of their power from the reader signal, the tags are only powered when in the beam of the reader 104. Class-3 and higher tags often include an on-board power source, e.g., a battery.
Because the tag reader is usually hardwired to a power source (e.g., power outlet), power efficiency is not generally a concern. Thus, class A amplifiers are typically used. However, as RFID becomes prominent as the tracking and identification system of choice, new RFID applications will be required. One of the most important of these is portable tag reader. As with any portable device, second only to reliability in importance is power consumption. However, the distortion inherent in non-linear amplifiers has been a barrier to use of class B and class C amplifiers in portable RFID transmitters.
To reduce the power requirements of portable RF transmitters such as portable RFID readers, it would be desirable to obtain the low power benefits of class C amplifiers with a clean, undistorted output.